This invention relates to power transmission belts of the rubber type, particularly a belt whose back surface possesses favorable frictional characteristics to allow it to make contact with and drive mechanical devices such as idler pulleys, tensioners, engine components such as a water pump, and the like.
Modern front end accessory drive systems for automobiles use serpentine multi-V-ribbed belts to link the engine crankshaft with various accessory driven pulleys. The backside of the belt typically impinges against backside idler pulleys and/or tensioning devices. Similarly, camshaft belt drive systems for automotive application systems use synchronous power transmission belts, the backsides of which are also trained about idler pulleys and/or tensioners. In addition, in many industrial belt drive applications, the drives include idlers or other devices against which the back of a multi-V-ribbed belt, synchronous belt, flat belt, V-belt or the like engages. In all of the foregoing automotive and industrial applications, in order for the backside of the belt to drive the device against which it impinges, the belt must have a minimum dynamic coefficient of friction, otherwise undesirable slippage between the belt and device occurs. For many automotive applications the minimum dynamic coefficient of friction has been set by the manufacturers at about 0.35. Positioning or adhering a textile fabric at the backside surface of the belt, due to the relatively low inherent coefficient of friction of the textile yarns making up the textile material, is inadequate to drive the mechanical devices off the backside of the belt, and does not meet the automotive manufacturers' specification.
The coefficient of friction on the backside of the belt can be increased, on the other hand, by calendaring the overcord textile material, such as tire cord, or a bias-laid fabric of the square woven kind, or in which the warp and weft yarns are oriented diagonally in respect to the longitudinal running direction of the belt, at an included angle between the yarns of about 90-120 degrees (so-called “Flex-Weave®” fabric-trademark of The Gates Corporation). The gum rubber that is applied during the calendaring operation fills the interstices of the fabric as well as presenting an outer layer of rubber on the fabric. This frictioned fabric/gum assembly is normally cut and respliced (using a Banner® table—a trademark of Burrowes Manufacturing Ltd) to provide the correct fabric cord orientation. This fabric cord orientation provides maximum or optimal lateral strength while allowing high flexibility in the longitudinal direction of the belt.
However, if calendared fabrics are used as the overcord fabric of the belt, in addition to inherently poor wear resistance of the outer rubber layer, most manufacturing processes require making overlapping splices to reconnect the material after “Bannering”, as well as during the belt building process. These overlap calendared splice joints create double thickness areas which have been found to cause noise and vibration in automotive serpentine drives. As the belt rotates around the drive, these splices contact the backside idlers, tensioners and the like which can cause the belt to emit noise and the belt and drive components to vibrate. Noise and vibration can also be caused as the backside idler, tensioner or other device makes contact with depressions left in the relatively thick rubber layer on the backside of the belt left as a negative impression from a polymeric film transfer label after the film, typically formed of a Mylar film (registered trademark of E.I. du Pont de Nemours) polyester backing, is stripped from the belt sleeve following vulcanization.
The use of knit overcord fabrics in rubber power transmission belts, per se, is known from U.S. Pat. No. 3,981,206 (Miranti et al). The knit fabric employs yarns made of a nylon-spandex biconstituent monofil. The knit fabric is bonded to the tension section of the belt with any suitable adhesive means. The belt construction of Miranti et al is built upright on a cylindrical drum carrying a matrix sleeve by applying various layers of material wrapped therearound, including the outer (nontubular) knit fabric. Such wrapping process will produce a seam or lapped joint.
Seamless knitted tubular fabrics have also been used in the overcord of non-rubber power transmission belts of the liquid cast (polyurethane) type. Unexamined Japanese patent application no. 7-243 483 (Bridgestone), published Sep. 19, 1995, discloses a multi-V-ribbed belt in which the tubular knit in the overcord is positioned directly against the tensile cord prior to liquid casting. The tensile cord and tubular knit make direct contact in the final fabricated liquid cast belt.
It is an object of this invention to overcome drawbacks in the prior art by providing a rubber power transmission belt utilizing a textile-reinforced overcord section which imparts lateral stability to the belt while allowing high flexibility in the running direction of the belt, and is characterized by use of a particular open mesh fabric construction which permits flow through of rubber during processing to achieve a belt back surface having optimum frictional and wear resistance properties.
It is a further object to achieve the foregoing using an overcord construction which is free of significant ridges or steps at the belt exterior surface which would generate unacceptable noise levels or vibration in belt drives using backside idlers, tensioners or other mechanical devices impinging on the backside of the belt.